1. Field of the Invention
The present invention relates to a hydraulic fluid change indicating device for an automatic transmission for a vehicle which can suitably indicate a warning of changing a hydraulic fluid for the automatic transmission.
2. Description of the Related Art
An engine oil used for lubrication of an engine for a vehicle and a hydraulic fluid used for lubrication of a transmission deteriorate with the elapse of time. Therefore, it is necessary to change the engine oil and the hydraulic fluid for the transmission. For example, it is recommended in an operation manual to change the engine oil every time a fixed distance is traveled or a fixed period is passed. The time of such engine oil change is shown by attaching a label at an easy-to-see place. However, such a periodic change of engine oil is apt to be forgotten.
If the engine oil or the hydraulic fluid for the transmission is used beyond an allowable deterioration degree, such an excess use may interfere with a lubricating operation. Accordingly, it is important to show a warning of changing the engine oil or the hydraulic fluid. However, the deterioration of the engine oil or the hydraulic fluid is dependent not only on the distance traveled or the period of time elapsed, but also largely on the use condition of a vehicle such as a road condition. Therefore, it is unsuitable to warn on the basis of only the distance traveled or the period of time elapsed.
For example, Japanese Patent Publication No. Hei 6-39893 discloses a lubricating oil change warning device having a configuration such that predetermined lubricating oil deterioration coefficients according to vehicle speeds and the uses of a lubricating oil such as an engine oil are preliminarily stored, a corresponding one of the deterioration coefficients is read according to a vehicle speed detected by a sensor and the temperature of the lubricating oil detected by a sensor, the degree of deterioration of the lubricating oil is computed by using this deterioration coefficient, and a warning of changing the lubricating oil is output when this deterioration degree exceeds a predetermined value.
The lubricating oil change warning device described in the above publication is mainly adapted to the change time for an engine oil. That is, the lubricating oil is an engine oil, and the vehicle speed and the temperature of the engine oil are used as physical quantities changing according to a vehicle operating condition. However, it is unsuitable to directly apply this lubricating oil change warning device to a hydraulic fluid change warning device for an automatic transmission. That is, the automatic transmission includes a torque converter, so that there is a special circumstance that the deterioration of the hydraulic fluid is promoted by heat generation in the torque converter. Accordingly, if the lubricating oil change warning device described in the above publication is used as a hydraulic fluid change warning device for an automatic transmission, a suitable warning of changing the hydraulic fluid cannot be given because a parameter on the heat generation in the torque converter is not considered.